Mass of the rocket doesnt stay constant

My physics class is ending the year with the study of rockets. He said it will be one of the most difficult studys of physics because the mass of the rocket doesnt stay constant. What do you think? Any hints/tips you would like to share? Thank you.

But seriously it shouldn't be that hard, you just have to factor in how much weight in fuel the rockets are averaging over 3 stages in flight (take off, level flight, descent) as each will be different. But, then we should also remember relativity, which states that mass increases as speed increases, and that size (in the direction of thrust) also becomes smaller.

But seriously it shouldn't be that hard, you just have to factor in how much weight in fuel the rockets are averaging over 3 stages in flight (take off, level flight, descent) as each will be different. But, then we should also remember relativity, which states that mass increases as speed increases, and that size (in the direction of thrust) also becomes smaller.